This invention relates to photographic objectives, and more particularly to photographic objectives having shortened total lengths.
Recently, as the minimization of the bulk and size of photographic camera advances, there is an increasing demand for a compact wide angle photographic objective by shortening the total length. However, as far as the wide angle lens system is concerned, the shortening of the total length generally leads to extremely large astigmatism and distortion. It is, therefore, quite difficult to design a lens system of shortened total length while still preserving a high image quality which is uniform throughout the entire area of the format.
To overcome this drawback and to obtain a good optical performance, a method is proposed in Japanese Patent Publication No. Sho 44(1969)-10831 where the front lens group is provided with a negative meniscus lens with strong curvature toward the rear and where the front surface of this meniscus lens is made aspherical. It is also known later to reduce the number of lens elements in the front lens group with the intention of minimizing the bulk and size of the objective as disclosed in U.S. Pat. Nos. 3,998,527 and 3,951,523.
On the other hand, Japanese Patent Laid Open No. Sho 54(1979)-76148 (U.S. Pat. No. 4,204,747) discloses an objective comprising a positive meniscus lens, a bi-concave lens, a bi-convex cemented lens, a negative meniscus lens concave toward the front, and a positive meniscus lens, the last meniscus lens being provided instead of the aspherical surface to facilitate the correction of distortion. In this five-component photographic objective, if focusing is performed by moving the positive meniscus lens to the cemented lens in unison, the aberrations will deteriorate. It is therefore pointed out in the specification that focusing provision is made at the cemented lens.
To achieve a shortening of the total length of the lens system, on the other hand, use may be made of the telephoto type in which a lens group of positive power is arranged in front of a lens group of negative power. In the case of the wide angle objective, however, since the limitation of the outer diameter of the objective to an appropriate value requires that the diaphragm be positioned at the middle part of the lens system with the lens group of positive power arranged on the front side of the diaphragm and the lens group of negative power on the rear side, such lens system tends to increase the difficulty of correction of the off-axial aberrations such as astigmatism and distortion.